Chlamydia trachomatis is the etiologic agent of trachoma, the world s leading cause of preventable infectious blindness. Trachoma is believed to be an immunopathologic disease which results from repeated exposure to chlamydial antigens. The chlamydial 60 kDa heat shock protein (GroEL) is the primary candidate for this immunopathologic antigen, although it remains controversial whether immune responses to Chlamydia-specific or highly conserved regions of GroEL lead to pathology. Moreover, it is unclear whether other chlamydial antigens contribute to immunopathology. This project will use a novel genetic approach to test the hypothesis that GroEL is responsible for the pathology of trachoma by identifying GroEl epitopes which elicit hypersensitivity in an animal model of ocular infection. This analysis will be extended to the study of recombinant chlamydiae expressing defined GroEL epitopes in the context of the whole organism. The long-term goals of this project include defining pathologic or protective GroEL epitopes to be excluded/included in a recombinant trachoma vaccine strain of Chlamydia. Perhaps more importantly, a reproducible system of gene replacement in Chlamydia will be developed and can be applied to the study of other chlamydial genes, providing unique insights into their function.